1. Field of the Invention
The present invention relates to a solid-state image sensor and a signal processing method thereof. More particularly, the present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor for use in, e.g., a camera-equipped mobile phone, a digital camera, or a video camera, and a signal processing method for calculation of a saturation signal level in the image sensor.
2. Description of the Related Art
As is well known, in charged coupled device (CCD) and CMOS image sensors, miniaturization of pixels has been advanced. At the present day, a cell having an area (cell size) of 2×2 μm or below, e.g., 1.75×1.75 μm or 1.4×1.4 μm has been developed. In such a development, a CMOS image sensor adopting a so-called vertical two-pixel one-cell structure to substantially increase an area of a photodiode (PD) has been already proposed to improve degradation in sensitivity of fine pixels (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 10-150182).
In general, when an area of a photodiode is reduced, a saturation electron number in an image sensor is greatly decreased. A decrease in the saturation electron number increases a ratio of shot noise that occurs due to a fluctuation of light (=√{square root over (electron number)}) and degrades the signal-to-noise ratio. Therefore, improving sensitivity of final pixels (improving the saturation electron number in the image sensor) has been demanded.
Here, the saturation electron number in the image sensor is usually set while considering a manufacturing process, an environmental temperature, a fluctuation in a power supply voltage, and others. For example, in case of a device that performs analog-to-digital conversion processing of 10 bits, a minimum saturation electron number in the worst operation environment is set as a saturation electron number in the sensor so that a digital signal on a 1023-LSB level as a maximum value of 10 bits can be obtained. The minimum saturation electron number in this case corresponds to approximately 60 to 70% of a saturation electron number with respect to conditions, a room temperature, and a standard power supply voltage near the center of a fluctuation in a general manufacturing process. The saturation electron number has particular large temperature dependence. For example, the saturation electron number is reduced at a high temperature of 60° C. (temperature characteristic compensation). Therefore, the saturation electron number of the sensor is set to a considerably low level. For example, the minimum saturation electron number is set to 6000 electrons in case of a device whose saturation electron number is 10000 electrons, and the same is set to 3000 electrons in case of a device whose saturation electron number is 5000 electrons.